1. Field of the Invention
The invention relates to the use of specific cycloalkylphosphines as catalysts for isocyanate dimerization (uretdione formation) and to a process for preparing polyisocyanates with a high uretdione group content.
2. Description of the Prior Art
Aliphatic isocyanates which contain uretdione groups and are based on optionally branched, linear aliphatic diisocyanates are distinguished by a particularly low viscosity. Products based on cycloaliphatic diisocyanates are generally highly viscous or solid substances which can be used as internally blocked crosslinkers, free from elimination products, in coating systems.
An overview of isocyanate oligomerization is given in J. Prakt. Chem./Chem. Ztg. 1994, 336, 185–200.
Tris(dialkylamino)phosphines (DE-A 3 030 513) optionally in conjunction with co-catalysts (DE-A 3 437 635) exhibit good selectivity for the formation of uretdione groups (uretdione selectivity). Their technical usefulness is hindered, however, by the serious imperfection represented by the high carcinogenic potential of their phosphorus oxides, e.g. hexamethylphosphoric triamide.
DE-A 3 739 549 discloses the catalytic NCO dimerization with 4-dialkylaminopyridines, such as 4-dimethylaminopyridine (DMAP), but the formation of uretdione is selected only in the case of specific cycloaliphatic isocyanates such as isophorone diisocyanate (IPDI). Linear aliphatic isocyanates such as hexamethylene diisocyanate (HDI) and also branched, linear aliphatic isocyanates such as trimethylhexane diisocyanate (TMDI) and methylpentane diisocyanate (MPDI), when used with DMAP and related compounds, give heterogeneous reaction products which are predominantly highly coloured.
DE-A 1 670 720 discloses the preparation of aliphatic polyisocyanates containing uretdione groups, in which the catalysts used are tertiary phosphines having at least one aliphatic substituent and also boron trifluoride and its adducts. It is noted that high fractions of uretdione groups in the product can be obtained only at low conversions and at reaction temperatures between 50 and 80° C., with the accompanying formation of isocyanate trimers (isocyanurates and iminooxadiazinediones) and also, particularly at a relatively high temperature, of other by-products such as carbodiimides or uretonimines. Uretonimines are especially disruptive since they tend to give off monomeric isocyanate in the course of storage.
The German patent application with the application number DE-1 025 487 8, hitherto unpublished at the priority date of the present specification, describes the use of phosphines containing at least one cycloaliphatic, P-attached radical as catalysts for NCO dimerization. The catalysts are distinguished by a substantially higher uretdione selectivity as compared with other trialkylphosphines of the prior art.